In DR1, we masked the inter-stack holes in each individual exposure before combining them ([1]}, Sect.3.1.3). However, in addition to producing lower S/N region in the final datacube, the masking was never perfect and left some artifacts. To improve the process, we build a superflat for each datacube, by combining many exposures where we cancel the rotation and dithering. This is done by adding a MUSE_SUPERFLAT_POS environment variable in the MUSE pipeline, which allows the RA, DEC, and DROT keywords to be overridden by the values of the exposure for which the superflat is built. This way the "instrumental grid" is the same for all the datacubes and the sources move because of the rotation and dithering of the field. As we have sparse fields, combining those exposures with sigma-clipping removes the astrophysical signal and produces the superflat, where only the instrumental residuals remain.
